Hydrated singly charged metal ions doped with carbon dioxide, Mg2+(CO2)-(H2O)(n), in the gas phase are valuable model systems for the electrochemical activation of CO2. Here, we study these systems by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry combined with ab initio calculations. We show that the exchange reaction of CO2 with O-2 proceeds fast with bare Mg+(CO2), with a rate coefficient k(abs) = 1.2 X 10(-10) cm(3) s(-1), while hydrated species exhibit a lower rate in the range of k(abs) = (1.2-2.4) x 10(-11) cm(3) s(-1) for this strongly exothermic reaction. Water makes the exchange reaction more exothermic but, at the same time, considerably slower. The results are rationalized with a need for proper orientation of the reactants in the hydrated system, with formation of a Mg2+(CO4)(-)(H2O)(n), intermediate while the activation energy is negligible. According to our nanocalorimetric analysis, the exchange reaction of the hydrated ion is exothermic by -1.7 +/- 0.5 eV, in agreement with quantum chemical calculations.